If You are sensitive to spoilers PLEASE do not read this.

Here's the strategy I used to beat Raihan: I had a LUcario and a Noivern (my Lucario knew Dragon Pulse) so I had them in the first and 2nd slots of my party. First comes Gigalith and the Flygon. I had my Lucario use Close Combat on the Gigalith (Rock is weak to Fighting) and I had Noivern use Draco Meteor on Flygon. I OHKO'd both of them. Then came the Duraludon and Sandaconda. I used Close Combat on Sandaconda and Dynamaxed Noivern and used Max Wyrmwind of G-max Duraludon. I OHKO'd the Sandaconda but only brought the G-max Duraludon down to Yellow HP. I had Lucario use Dragon Pulse on G-max Duraludon and had Noivern use Max Wyrmwind which KO'd it. Basically I took advantage of it being a double battle and both pokemon knowing a strong Dragon type move.